This invention relates to a method of improving process control in the manufacture and purification of vinyl acetate, and a method of manufacturing vinyl acetate utilizing improved process control.
The prevailing method of vinyl acetate production is a vapor phase process involving continuously reacting ethylene, oxygen and acetic acid in a fixed bed catalyst reactor. The catalyst, generally palladium or a palladium/gold mixture is supported on a silica or alumina base. The two principal reactions that occur in the vinyl acetate process involve the reaction of ethylene, acetic acid and oxygen to form vinyl acetate and the undesired combustion of ethylene to form carbon dioxide and water. Other impurities formed in the production of vinyl acetate include acetaldehyde, ethyl acetate, methyl acetate, acetone, glycol diacetate, acrolein and crotonaldehyde.
The selectivity of the process and the percent conversion of the reactants are a function of several variables including reactor temperature, desired productivity and the condition of the catalyst. Deactivation of the catalyst, which routinely occurs over time due to buildup of tars and polymeric materials on the catalyst surface, can adversely affect the reaction process, particularly with regard to selectivity. These changes in reactor performance can ultimately lead to compositional changes in the liquid stream entering the purification section of a vinyl acetate plant.
In producing vinyl acetate, the reactor products of vinyl acetate, water and carbon dioxide are separated from the raw materials of ethylene and acetic acid, which are used in excess. The ethylene and acetic acid are recycled back to the reactor from the reaction and purification sections of the unit. Product vinyl acetate is recovered and purified in the purification section and sent to storage tanks. Wastewater is sent to a treatment facility and carbon dioxide is vented to a flare for disposal. Inert gases, such as nitrogen and argon, are typically purged from the reaction section to minimize buildup. These inert gases may or may not pass through an ethylene recovery unit installed to reduce loss of ethylene associated with the purge.
It is known to monitor the reactor feed and effluent by on-line gas chromatography and mass spectrometry. These on-line techniques are generally backed up by daily samples to cross check on-line analysis. Monitoring the reactor feed and effluent has a two-fold purpose. The first purpose is safety. The high flammability of ethylene and oxygen require that the feed composition be carefully monitored to ensure that the explosive limits are not exceeded. The second purpose is reactor performance. It is important to monitor the selectivity of the process and the percent conversion of the reactants, so that any adverse changes in these parameters can be quickly addressed.
A gas chromatograph (GC) can analyze the inlet and outlet reactor streams for carbon dioxide and ethylene. The carbon dioxide concentration is used to judge reactor performance and to calculate reactor selectivity. While this technique is adequate, it does have a number of drawbacks. A sample system is required to deliver the sample to the GC, and there is potential for chemical changes to occur in the sample transfer lines prior to analysis. Also, typically at least two separation columns must be used to separate the various stream components prior to GC analysis. A first column would typically be used to remove acetic acid and vinyl acetate from the sample. A second column would then be used to separate and quantify carbon dioxide and ethylene. Because of the need for a series of columns, typically a complete GC analysis requires at least fifteen minutes or so leading to a significant time lag between real time composition and measured composition. Thus, measurements are generated infrequently, and the number of hardware items involved in the analysis increases the potential for maintenance.
A mass spectrometric analyzer can be used to determine the percent composition of reactor feed and effluent components. As in the case of gas chromatography, a complex sample system is required to deliver the sample to the analyzer, resulting in infrequent measurements, and the potential for chemical changes occurring in the sample lines, incomplete sample delivery, and other related problems.
Persons skilled in the art of vinyl acetate processing are aware that undesired cycling or pulsing of reactor inlet and outlet component concentrations can occur. This cyclic behavior would generally be in response to downstream upsets that cause such pulsing to occur in recycle streams that feed the reaction system. An example would be a cyclic variation in the temperature of the primary tower bottoms recycle to the reaction section. Depending on the cycle frequency, it is possible that this undesired behavior will go undetected with an inadequate frequency of analysis, such as the time frame of fifteen minutes or greater achievable with gas chromatographic analysis and mass spectrometric analysis.
On-line infrared spectroscopy has been used for characterizing and quantifying components of a chemical process gas or liquid stream. For example, the use of on-line infrared analysis in controlling reactor liquid composition in the acetic acid process has been described in U.S. Pat. No. 6,103,934 entitled MANUFACTURING AND PROCESS CONTROL METHODS and U.S. patent application Ser. No. 09/611,067 filed Jul. 6, 2000 and entitled MANUFACTURING AND PROCESS CONTROL METHODS. The advantages of infrared spectroscopy in analysis of an acetic acid manufacturing process downstream of the reactor were discussed in copending U.S. patent application Ser. No. 09/672,893 filed Sep. 29, 2000 and entitled PROCESS CONTROL FOR ACETIC ACID MANUFACTURE. The use of on-line infrared analysis as described in Ser. No. 09/672,893 provided real time process control of component concentrations in a reaction system for producing acetic acid.
Similar to the manufacturing process for acetic acid, production and purification of vinyl acetate requires removal of other components from the vinyl acetate product and, where necessary, to either recycle these other components (acetic acid, water, ethyl acetate, acetaldehyde, polyvinyl acetate, ethylene and carbon dioxide) to the reactor or other parts of the process, or send these other components to waste with minimum product or raw material loss. The composition of these purification streams will partially be a function of reaction section performance and partially a function of purification section column performance. There is thus a need to implement process control via on-line infrared analysis in the reaction system for the production of vinyl acetate.
The present invention provides a method of real time process control of component concentrations in a reaction system for the production of vinyl acetate from the oxidation of ethylene and acetic acid. To this end, and in accordance with the present invention, samples of reaction system solution are collected from the reactor vessel feed and/or effluent and/or from columns and/or transfer lines downstream of a reactor vessel, and the concentration of one or more components in the sample is measured by an infrared analyzer. The concentration measurements are used to make adjustments in the process. The concentration of one or more components is adjusted, either directly or indirectly, in one or more locations in the reaction system in response to the measurements. For example, the flow rate of a solution stream in a transfer line can be increased or decreased going into or out of a column to alter the concentration of one or more of the components in that column or another vessel in the reaction system. Alternatively, the temperature of the solution in a column or stream or the temperature profile or gradient in a column could be increased or decreased to affect the concentration of one or more components in the reaction system solution. Also, the concentration of a reaction system component can be adjusted by direct addition or extraction of that component into or out of the solution. For example, acetic acid concentration in the reaction system can be adjusted either directly by increasing or decreasing the acetic acid feed into the acid tower that feeds the reactor vessel, or indirectly by increasing or decreasing recycle stream flow rates containing acetic acid to the reaction section. Thus, reaction system component concentrations can be adjusted directly or indirectly by varying any number of process variables in the reaction system. Further, adjustment in one location of the reaction system may cause concentration changes at either that location or upstream or downstream of that location. For optimum process control, the measurements are transmitted to a control unit for real time analysis, and the adjustments are made almost instantly after the infrared analysis. There is thus provided a method for continuously updating the conditions of the reaction system to enhance process control in real time of the overall process to thereby optimize the production and purification of vinyl acetate product.